The state of the art already comprises much equipment of this sort, among which the following:
Applicant's French patent 2,410,081 comprising, within a closed tank, a cylindrical screen and a closed cylindrical rotor fitted at its periphery with blades sweeping the screen at high speeds while very close to the perforated sheetmetal. This equipment may operate either centrifugally or centripetally depending on the rotor's direction of rotation. The perforations are holes or slits depending on the nature of the pulp and impurities and the inner screen surface is smooth or contoured. in the latter case the perforations being located at the groove bottom.
The blade assumes the shape of a wedge with a radial edge and an edge sloping at 20.degree. to 30.degree. relative to the rotor wall. Whatever the direction of rotation, the pulp between the rotor and screen is driven into rotation with the rotor. For centrifugal operation, the radial surface perpendicular to the rotor is similar to a scraper pushing the pulp.
Several equipment of this sort are used in cascade in many plants, though individually such equipment will differ by their centrifugal/centripetal operation. Preferably, the coarse pulp from the primary disintegration treatment by pulpers will be filtered in this equipment set for centrifugal operation. The coarsest impurities (threads, nettings etc.) are then eliminated. The pulp only containing thereafter impurities of small diameters is then filtered in identical equipment set for centripetal operation.
When used in the centrifugal mode, this equipment operates at concentrations about 3 to 4% but this operation is accompanied by substantial fractionation. The pulp rotating at a speed close to that of the rotor aligns the fibers in the direction of motion in a plane parallel to the screen surface. These aligned fibers (the longest) mix with the threads and nettings and other impurities and are eliminated with the discards.
U.S. Pat. No. 3,726,401 discloses a centrifugal purification apparatus comprising a screen inside which turns a rotor constituted of eccentric lobes (relative to the longitudinal axis of the screen); illustratively the lobes are mutually eccentric, the cylinder being oval in cross-section.
The object of this rotor shape is to generate pulses precluding screen clogging: that portion of the rotor approaching the screen generates a pressure increase while the portion moving away from it causes a depression (partial vacuum).
Such a shape is well known in pump technology and entails a substantial rotation of the pulp.
In order to avert excessive fractionation, the pulp is recirculated through the hollow inside-space of the rotor. Accordingly the pulp undergoes several motions past the screen perforations.
The European patent document 206,975 is a centrifugal purifier comprising a closed rotor and designed, in the manner of that of the previous document, to be based on the lobes of an eccentric cylinder; however instead of being mutually eccentric excentric, they are sliding in mutually excentric manner whereby two cross-sectionally offset half-cylinders are created. This rotor rotates in front of a contoured screening surface similar to that described in the French patent 2,410,081. The operating principle is the same: the rotor comprises two opposite shoulders extending over the entire length of the screen and serving to drive the pulp at the rotor speed. The shoulders are scrapers. However, differing from the scraper of the above cited equipment, the rear side of the shoulder is neither planar nor sloping, instead it is cylindrical and thereby the depression generated will be less and spread over a larger peripheral surface.
The high speed imparted to the pulp endows it with a high centrifugal force contributing to higher screening outputs which are however little affected by the said depression.
While this equipment is satisfactory with respect to purification outputs, other hand it incurs the drawback of high fractionation which is merely limited by the screen contour.
Moreover, this equipment is power-intensive to set the pulp in motion. It also, requires using a screen highly resistant to mechanical stresses because of knocking from the shoulders parallel and symmetrical relative to the generatrix; the overpressures and depressions are diametrically opposite and the opposite deformations are additive. These stresses add to costs which may be quite substantial in case of malfunction.
It follows from the above description that the above purification equipment entail the main drawback of more or less substantial fractionation which is reflected both by material losses, and moreover the loss of the most valuable material, namely the long fibers used for high-grade paper manufacture.
The main object of the invention is to create equipment avoiding this fractionation.
Fractionation appears to arise mainly from the pulp rotating at a speed near that of the rotor and it is averted in centrifugal purifying equipment using open drums; such equipment is well known (for instance French patent 1,546,515) and consists of a cylindrical screen inside which a rotor rotates about its axis. This rotor consists of an axial shaft bearing arms to the ends of which are affixed cross-sectionally hydrodynamic blades.
Regarding such, i.e. the conventional purifying equipment, the pulp filling all the cylinder occupied by the rotor is made to move near the screen by the blade rotation. However, this motion is substantially less than that of the blades. and this feature is required to pulse the pulp when the blade is moving by.
The fibers subjected to such pulses cannot align themselves in the flow path of the pulp, and thereby fractionation is averted or lessened. However this purification by means of a drum with "open" rotor is unsuitable for the first purifications of very soiled pulps, nor is it suitable for high concentrations because. in spite of the pulses, the screens will rapidly clog, foremost because the impurities and the long fibers will straddle the leading edges of the blades.